Five days after humbling Barcelona 4-1 in the Champions League, the French champions lacked stamina and inspiration as they fell four points behind leaders Lille who have 58 points from 26 games after beating Lorient 4-1 on Sunday.
Kylian Mbappe, who impressed with a hat trick at the Camp Nou on Tuesday, struggled against a compact Monaco defence and the visitors took the lead after only six minutes when Diop headed home.
PSG increased the pressure before the interval but failed to muster a single shot on target.
Niko Kovac's side doubled their advantage six minutes into the second half when centre back Maripan fired home from inside the area after the PSG defence had failed to clear a cross.
Verratti came on for Ander Herrera in the 55th but the Italian had little influence.
"It's another missed opportunity in the title race. We need to understand that there are good teams in France, too. It's not just about the Champions League," Verratti said.